By selecting animals on their breeding value calculated mainly from phenotypic measurements of production traits, breeding has greatly improved the genotype for production traits of livestock animals. Thus, traditionally, breeding programmes have selected for phenotypic characteristics of animals. However, more recently selection for genotypic characteristics that are associated with improved production traits have gained interest in the field. Selection for phenotypic characteristics entails mainly selection of the offspring or siblings or other relatives of the animals to be selected whereas selection of specific genotypic characteristics allows for earlier and specific detection of animals of interest.
Within methods that select on specific genotypic characteristics, one may distinguish between methods that detect genetic variation in genes or quantitative trait loci that are merely associated with production traits of animals and methods that detect genetic variation in functional genes that directly influence those production traits. One of the former methods is a marker assisted selection wherein polymorphisms in markers identified in a random manner are associated with production traits.
For instance, meat production is closely linked to embryonic muscle formation. Biologically, production is concentrated in defined tissues of the animal, e.g. muscle tissue for lean meat production. In breeding programmes for optimizing porcine lean meat production, various levels of selection pressure have been applied to different tissues (i.e. muscle, fat and bone).
Muscles are complex tissues composed of a number of different cell types, e.g. myocytes (the most predominant) consisting of myofibres and satellite cells, intramuscular adipocytes, fibroblasts, endothelial cells, neurocytes, etc. Handel and Stickland (1984, 1988) showed that the number of myofibres present at birth determines the maximal lean meat growth capacity of pigs. Double-muscled cattle show a higher number of prenatally developed myofibres than other cattle (Swatland and Kiefer 1974; Hanset et al., 1982), which suggests that lean meat production capacity is determined by the embryonic development of myocyte number.
Myogenesis is a complex, multistep process that chronologically involves:
(1) Progenitor cell determination to the myogenic lineage. PA0 (2) Migration of myogenic stem cells (myoblasts) to appropriate locations in the early embryo. PA0 (3) Proliferation of myoblasts and non-myogenic muscle-tissue cells. PA0 (4) Terminal myocyte differentiation (i.e. fusion of myoblasts) and expression and organization of specific gene products active only in terminally differentiated muscle cells. PA0 (5) Maintenance of the terminal differentiated state and modulation of myofibres in various myofibre types in response to age and physiological cues (Edgerton and Roy, 1991; Funk et al., 1991; Gunning and Hardeman, 1991; Olson, 1992).
A model based on the action of the MyoD gene-family describes a mechanism for the genetic regulation of myogenesis. There are four members of this family in vertebrates, MyoD (also called Myf-3), myogenin (Myf-4), Myf-5 and MRF4 (Myf-6, herculin). A number of recent reviews summarizes in detail the existing knowledge of the structure of the genes, the MyoD-myogenesis model and the activation of muscle tissue-specific genes by the MyoD genes (for reviews see above).
MyoD proteins are expressed specifically in muscle tissue where they act as tissue-specific transcription factors. In vitro they are active after formation of dimer complexes with proteins of the ubiquitously expressed E2A gene. The complex binds to specific transcription regulatory sequences of muscle-specific genes called enhancer regions in the promoters, thereby activating expression of the tissue- and developmental stage-specific genes like muscle-specific actin, tropomyosin and titin (reviewed in Olson, 1990; Weintraub et al., 1991; Lyons and Buckingham, 1992).
Once activated, each member of the MyoD gene family can both positively autoregulate its own expression and regulate the expression of other MyoD genes in differentiating in vitro muscle cell cultures, thereby continuing the differentiation pathway. Thus, once the pathway is activated, myogenesis continues until terminal differentiation is established.
Determinated cells (myoblasts) are able to migrate (step (2) of the myogenesis pathway) and profilerate (step (3)); Olson, 1990, 1992). Irreversible terminal differentiation (step (4)) is induced by fusion of the myoblasts into multinucleated myofibres. The fusion is induced by the activation of the myogenin (Myf-4) gene in myoblasts (Olson, 1990, step (4)).
The research for genetic variation within the MyoD genes is already underway in a number of laboratories. Recently the first polymorphism was reported in the myf-4 gene in pigs (Ernst et al., 1993). Allele freqencies differed between different pig breeds. It is still unclear where in the gene this polymorphism is located and whether this polymorphism could be related to different muscle growth potentials and be used as a marker within selection lines of one breed.
Myogenin is the only myf gene expressed in all skeletal muscle cell lines (Wright et al., 1989; Edmondson and Olson, 1989). Knock out experiments in mice have shown that this protein fulfils an essential function in muscle differentiation by regulating the onset of myoblast fusion and the formation of functional muscle fibers (Hasty et al., 1993; Nabeshima et al., 1993).
RFLPs have been described for the mouse MyoD gene (Kay et al., 1993), the bovine myf-S gene (Dean et al., 1993) and the pig myogenin (Ernst et al, 1993) and myf-6 genes (Ernst et al., 1994). One MyoD allele in mouse seems to be associated with the increased efficiency of muscle regeneration of the SJL/J strain.